Method of etching and solution therefor



' 3,436,283 METHOD OF ETCHING AND SOLUTION- THEREFOR William A. Chrisley, 720 Twin Hills Drive, El Paso, Tex. 79912- No Drawing. Filed Apr. 23, 1965, Ser. No. 450,515

Int. Cl. C231. 1/02 US. Cl. 156-14 3 Claims ABSTRACT OF THE DISCLOSURE An etching solution for photoengraving containing typically 5 to 18% by volume nitric acid, a water insoluble organic liquid substantially unreactive with the nitric acid, a cationic emulsifying agent in an amount not in excess of 1% by weight of the solution, and ,a non-ionic emulsifying agent in .an amount of not more than 1% by Weight of 'the solution.

the metal surface, and then exposing the sensitized surface to 'a light pattern through a photographic negative.

,The latent image thus produced is then developed with the application of a resist coating, the coating being hardened by heat. The resulting hard coating is then etched, typically by nitric acid. Great care must be exercised in the use of such nitric acid solutions, however, to avoid etching under the image area, thereby weakening these areas by undercutting.

Various additives have been suggested in nitric acid baths to remedy these defects, but few of these have been acceptable commercially. In some cases, the additives tended to inhibit-the etching of the nitric acid too severely with the result that long times and repeated applications were necessary. In other cases, etching was rendered difficult by an effect which is apparently a chelation of the metal ions in the solution. The need still remains, therefore, for an efiicient etching method which will permit controlledetching of the plate by the nitric acid solution without side effects caused by interaction of the various reagents in the etchant. The satisfaction of that need is the principal objection of the present invention.

Another object of the present invention is to provide an improved etching solution containingnitric acid and a combination of emulsifying agents which permit the etching to occur rapidly but without undercutting the image areas on the resist coating.

A further object of the present invention is to provide an improved method for etching photoengraving plates with an etchant containing controlled amounts of both a cationic and a non-ionic emulsifying agent.

The etching composition of the present invention includes nitric acid in amounts substantially equivalent to concentrations heretofore used for that purpose, in combination with a water insoluble, organic liquid which is unreactive toward the aqueous nitric acid solution. Typically, this unreactive liquid is a petroleum fraction, straight chain or aromatic, having a boiling range broadly from 150 to 730 F. These fractions may include materials which are normally classified as gasolines, benzine, kerosene, lubricating oil and the like.

For best results, nitric acid is added in amounts such that the final bath has between 5 and 18% by volume nitric acid, calculated on the basis of a commercial nitric Patented Apr. 1, 1969 acid solution containing 65% HNO In other words, the final bath has about 5.2 to 9.75% by weight of the compound HNO The organic liquid usually comprises about 1 to 15% by weight of the final bath, and preferably from about 2 to about 10% by weight. Typical petroleum fractions which can be used for the purpose of the present invention are those identified commercially as Varsol which is a straight petroleum aliphatic solvent having a minimum Tag closed cup flash point at F., or Solvesso which is an aromatic solvent prepared from petroleum.

The improvements attributable to the etchant of the present invention are due to the joint use of a cationic emulsifying agent and a non-ionic emulsifying agent in the etchant both emulsifying agents being present in amounts no more than 1% by weight each. The use of the two agents jointly apparently provides a synergistic effect which promotes etching by the nitric acid even in the presence of metal chelation and corrosion inhibition.

The preferred concentration of the cationic agent is in the range from about 0.05 to about 1.0% by weight. While all cationic surface active agents having suitable solubility characteristics can be used for the purposes of the present invention, the most important class consists of the quaternary salts of which the following is a generic expression:

where R is a straight chain aliphatic group of 12 to 20 carbon atoms, a, b, and c are methyl, ethyl, benzyl, or hydroxyl; X is Cl, Br, I, methosulfate, or ethosulfate.

A typical example of a quaternary ammonium salt useful for the present invention is that available under the trade name Penetronyx which is an alkyl dimethyl hydroxy ethyl ammonium chloride.

Another type of quaternary surfactant which can be used is an alkyl pyridinium halide. Still others are the quaternary morpholinium alkyl sulfates which are prepared by reacting a higher fatty primary amine with dichloro diethyl ether and then reacting with lower alkyl sulfates.

Still another type of cationic material which can be used for the purposes of the present invention consists of the fatty acid derivatives of N-hydroxy ethylene diamine containing 8 to 22 carbon atoms in the fatty acid portion.

In addition to the quaternary compounds, non-quaternary nitrogen bases can also be used. Such compounds include the fatty nitriles, the cationic amines such as the fatty acid derivatives of propylene diamine, the N-hydroxy ethyl alkyl amines, and the addition products of ethylene oxide with primary and secondary amines (Ethomeens).

Other classes of cationic surfactants include amines with intermediate linkages such as the esters of ethanolmorpholine, ether linked amines of the type having the formula ROCH CH CH NH and amide linked amines of the type RCONHC H NH where R is a fatty acid residue having about 8 to 22 carbon atoms.

Another important class of cationic surfactants for the purpose of the present invention are the imidazoline derivatives made by condensing a fatty acid with ethylene diamine or hydroxy ethylene diamine (such as the commercial product Onamine RO) The non-ionic material is present in amounts not in excess of 1% by weight, and preferably is included in amounts ranging from about 0.009 to 0.06% by weight. Suitable non-ionic emulsifiers include the commercial material known as Neutronyx which is an alkyl phenol polyglycol ether containing about 9 mols of ethylene oxide per molecule of the emulsifying agent. Still other 3 suitable materials are the esters of methoxy polyethylene glycol esterified with stearic acid, soybean acids, oleic acid, coconut oil acids, and lauric acid.

A wide variety of non-ionic surfactants suitable for use in the present invention is available under the trade name Tergitol of which the following members are representative:

Tergitol nonionic NPX NP-27 Alkylphenyl ether of polyethylene glycol NP-35 12P-6Dodecyl phenol adduct with 6 mols of ethylene oxide l2P-12Dodecyl phenol adduct with 12 mols of ethylene oxide TMN-3 Trimethyl nonyl ether of polyethylene TMN-lO glycol XDPolyalkylene glycol ether A typical bath formula is given in the following table:

Total bath liters 80 Onamine R cc 700 Neutronyx cc 30 Solvesso 150 cc 2100 Nitric Acid (65%) liters 11.5

Balance, water.

In the application of the etchant, it is desirable to splash the etchant against the surface being treated repeatedly. The nitric acid thereupon attacks the metal between the resist areas, wih the combinaion of the two emulsifying agents permitting etching through the combined effects of metal chelation and corrosion inhibition. Apparently, the cationic material reacts with the acids present to form water soluble cationic salts which assist in the etching, while the non-ionic emulsifying agent controls the wetting ability or viscosity of the cationic compound. In any event, the combination of the two emulsifying agents provides advantages not possible with the use of each individually.

It should be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

I claim as my invention:

1. The method of controllably etching the surface of an acid soluble metal photoengraving member which comprises contacting-the surface to be etched with an equeous solution containing from 5 to 18% by volume nitric acid, from about 0.05 to about 1.0% by weight of a cationic emulsifying agent selected from the group consisting of quaternary ammonium salts, fatty acid derivatives of N-hydroxy ethylene diamine containing from 8 to 22 carbon atoms in the fatty acid portion thereof, fatty nitriles, fatty acid derivatives of propylene diamine, N-hydroxy ethyl alkyl amines, addition products of ethylene oxide with primary and secondary amines, and fatty acid derivatives of imidazoline, and from about 0.009 to 0.06% by weight of a non-ionic emulsifying agent selected from the group consisting of alkyl phenol polyglycol ethers, fatty acid esters of methoxy polyethylene glycol, and polyalkylene glycol ethers.

2. The method of claim 1 in which said organic liquid is a petroleum fraction.

3. An etching solution for a metal photoengraving member comprising an aqueous solution containing from 5 to 18% by volume nitric acid, a water insoluble organic liquid substantially unr'eactive with the nitric acid, from about 0.05 to about 1.0% by weight of a cationic emulsifying agent selected from the group consisting of quaternary ammonium salts, fatty acid derivatives of N-hydroxy ethylene diamine containing 8 to 22 carbon atoms in the fatty acid portion thereof, fatty nitriles, fatty acid derivatives of propylene diamine, N-hydroxy ethyl alkyl amines, addition products of ethylene oxide with primary and secondary amines, and fatty acid derivatives of imidazoline, and from about 0.009 to 0.06% by weight of a non-ionic emulsifying agent selected from the group consisting of alkyl phenol polyglycol ethers, fatty acid esters of methoxy polyethylene glycol, and polyalkylene glycol ethers.

References Cited UNITED STATES PATENTS 3,023,138 2/1962 Easley et al. 1'56-'l4 3,152,083 10/1964 Croisant et al 25279.4 3,312,625 4/1967 Peterson 252149 3,320,171 5/1967 Fishaber 25279.4

JACOB H. STEINBERG, Primary Examiner. 

